Summer Simmer

Tom Vanderbilt

As I write, the temperature in New York City is 86° F. The relative humidity is 56, the winds are south-westerly at seven mph, visibility stands at six miles. What do those numbers really signify? The temperature doesn’t sound extreme, yet when I leave my air-conditioned house I don’t feel that I’m stepping outside so much as entering another atmosphere. My spirits sag, my lips soon taste of sweat. Is this 86°? Well, yes, because that’s what the New York Times says it is. On the other hand, as any schoolchild knows, what really matters is the humidity, and the increasing capacity of air to retain moisture as its temperature rises.

On another index, the trademarked ‘RealFeel’ reading provided by AccuWeather, the current temperature is actually a sweltering 96°. According to AccuWeather, Real Feel is ‘the first ever temperature index that takes into account multiple factors in determining what the temperature outside feels like’. Here empiricism begins to fray at the edges. If the temperature feels like 96°, then why is it 86°? What will it ‘feel like’ when it really does climb to 96°? To complicate matters, the RealFeel temperature can be the same as, or even lower than the ‘actual’ one. We are accustomed to the relativity of numbers – the dollar is a fixed standard, yet according to the economic climate it will buy more or less – but this variation in a seemingly simple, everyday measure speaks to the problems of quantifying abstract experience.

The 20th century was the century of standardisation. There were initially any number of scales for measuring weather – Gustave Eiffel, for example, used his tower as the test site for an array of barometric and other climatic indices – but just as time was eventually standardised to meet the needs of the railroads and the telegraph, so the measurement of weather was gradually reduced to a few benchmark indices. One of the first significant refinements made to the work of Messrs Fahrenheit and Celsius was the invention of the wind chill index in 1945 by two geographers working in Antarctica, who measured how long it took for the water in a series of plastic cylinders to freeze. The original wind chill index was measured in units that corresponded to the rate of heat loss; one of the geographers, Paul Siple, later regretted this, as the use of units was often taken to imply that the index was an alternative temperature scale. This past winter a US-Canadian team unveiled a new wind chill index without the Celsius figure attached. In time, those numbers will assume a meaning of their own, but for the moment they are raw, as meaningless as g-force measurements to the average non-astronaut.

Now, for the summer, we have the heat index, first proposed in 1979 by the Australian environmental scientist Robert Steadman in an article in the Journal of Applied Meteorology entitled ‘The Assessment of Sultriness Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science’. Steadman’s index depends on two variables – temperature and humidity – and a number of fixed hypothetical assumptions concerning clothing, physical activity, time of day and heat loss due to radiant heat transfer and evaporation. It was never the product of a single true formula, however, but of a collection of formulae. What we now call the ‘heat index’ is the US National Weather Service’s best approximation of what those formulae actually mean – the ‘apparent temperature’. There are various other indices: the ‘summer simmer index’, proposed in 1987, produces a temperature equivalent that indicates how hot a particular condition would feel in a desert setting with relative humidity of 10 per cent. Using that index, another meteorologist produced a list of the ten American cities with the worst summer weather conditions (happily presupposing what ‘worst’ means). Most are in Texas, and the absolute worst is Corpus Christi (a place once risibly proposed to tourists as ‘the Italy of America’), which from May to September has an average summer simmer index temperature of 100.1° F.